Hi Mark,
I past days I have always worked at places that employed electricians so didn't really take much notice apart from the basics. Times are a bit different now and the basics are a bit more complex so I have had to learn a lot in the last few years.
Yes, I do understand your explanation. I had it in my head for some reason that the earth was direct to the alternator body but giving it some thought, of course it must go through the rotor or else it would not produce emf (to the lamp) when charging.
Many thanks,
Bryan.